Began with incenter.html, the first one alphabetically. Needed one
new point construction method, and a new option to see what was
going on.
Got the planar diagrams on that page working. The next step on #36 will
be to get 3D diagrams as the theorem on this page generalizes to 3D. That
will be a bigger task, so merging this now.
Reviewed-on: #39
Co-authored-by: Glen Whitney <glen@studioinfinity.org>
Co-committed-by: Glen Whitney <glen@studioinfinity.org>
